1. Field of the Invention
This invention relates generally to devices which detect a buried or concealed conductive structure, and particularly to one which enables the determination of its depth or interruptions in it.
2. Background of the Invention
There currently exist, and are commonly used, many electronic devices for locating diverse buried or concealed power cables, telephone cables, television cables, and numerous other structures. Virtually all such locators employ the principle of injecting, by direct connection or by induction, an alternating current of relatively high frequency into the structure to be traced and then following the course of the radiated signal from the structure with a suitable receiver containing an indicating means, such as a loudspeaker, meter, digital display, or other device. One such device, the Pipehorn.TM., is manufactured and sold under U.S. Pat. No. 3,418,572, issued Dec. 24, 1968.
In addition, there now exists a number of devices particularly intended to determine the depth or distance to such concealed structures. Most such new devices, including one disclosed in U.S. Pat. No. 3,893,025, issued July 1, 1975, employ two vertically displaced antennas and measure depth and utilize the difference in the signal level detected by the two antennas and relate the difference, mathematically, to the distances involved. Actual readings are generally from a scale on a potentiometer, sound nulls, or by digital display. Unfortunately, the usefulness and accuracy of such devices is constantly being eroded by the installation of greater numbers of energy delivery systems, communication cables, lighting system cables, and other diverse structures, most of which are conductive. As a result, the applicants have concluded that as a practical manner, the readings from two antenna systems lack sufficient accuracy for serious usage for determining the depth of a conductive structure.
In addition to the problem of the determination of the depth of a conductive structure, there exists a second problem related to buried conductive structures, and that arises with respect to ferrous pipe carrying natural or manufactured gas or other combustible hydrocarbons, such as gasoline or propane or any other hazardous material, as these structures are required to be cathodically protected by application of a direct current such that electrical current will flow from soil around the pipe to the pipe and thus mitigating, preventing, opposite directional flow and thus electrolytic corrosion of the pipe. For many years, the most common method of supplying such current has been to utilize power from electric utilities and suitable transformers and rectifiers to provide the direct current at suitable voltage and current levels to sustain the desired potentials over the run of the pipe. In order to minimize potentially destructive stray currents or on other structures, many owners of such systems are now using a "distributed ground bed" technique wherein anodes are installed at spaced intervals along and adjacent to the pipe to be protected, with a continuous insulated cable connecting the anodes. With such a system, it is extremely important to determine the location and condition of the anodes and connecting cables. It is also extremely important to be able to trace the flow of the current in the various structures and to determine its relative magnitude at specific points in order to locate defects in the system, particularly breaks or short circuits which will prevent continuous coverage.
As far as is known by the applicants, the only effective and practical way of testing a cathodic protection system is simply the direct one of actually measuring the potential on elements of the system with respect to surrounding earth. This, of course, means that one has to do a tremendous amount of digging, and thus it is apparent that there is a substantial need for a system which will enable above-ground survey and location of defects in such a system.
Accordingly, it is the object of this invention to provide an above-ground, conductive-structure locating device which, after generally locating the region in which such a structure lies, will accurately indicate its depth, and where required, determine the location of defects.